1. Field of the Invention
The invention relates to a method and to a device for evaporating large quantities of metals and semiconductor materials by electromagnetic levitation, in which the levitating evaporand is heated to evaporation by electrical means and vapor condensate on parts of the electrical means or on screens thereof is caused to melt off so that melted off condensate is deposited into the levitating evaporand by electromagnetic field forces acting upon it.
2. The Prior Art
It is known that large quantities of metals such as Al, Ag, Au, etc., or metallic compounds such as CrNi, or semiconductors such as Si can be evaporated from web coating, plastic and metal parts coating, etc. onto moving or stationary targets. The evaporand is usually contained in one or several resistance heated boats or crucibles, in particular to boat-shaped slabs provided with a cavity and made of a high melting point metal or of a mixture of high melting point inorganic compounds. The evaporand placed into the boat or crucible is first melted, and then brought to evaporation temperature, the latter usually exceeding by several hundred degrees C the melting point of the evaporand.
As an example, Al has a melting point of 660.degree. C., the boats used for its evaporation are usually made of sintered AlN+BN+TiB.sub.2 and are heated to 1450.degree.-1500.degree. C. in commercial web coating units. Ag melts at 961.degree. C., Au at 1063.degree. C., the crucibles or boats made for their evaporation are usually made of W and have to be heated to approximately 2000.degree. C. in order to achieve sufficiently fast evaporation for industrial use.
The high temperatures required for industrial evaporation result in a relatively short life of the evaporators used. They usually do not last more than 15 to 20 hours of effective evaporation. The number of hours could be increased by decreasing the temperature by several hundred degrees; however, lowering the temperature also decreases the speed of evaporation below that required by the industry. Nor can--as often required--efficient evaporation take place under these conditions at pressures exceeding a few tenths of a micron.
Induction heated crucibles of graphite are also being used, their inner wall coated with BN or TiC. Both coatings offer a temporary protection only against the corrosive action of reactive metals such as Al heated to evaporation temperature. Graphite crucibles also have the disadvantage that the co-evaporating carbon produces the undesirable, foul smelling, hygroscopic compound AlC in the condensed Al layer. It has therefore been tried to replace graphite crucibles with ceramic crucibles made of such electrically non-conducting materials as AlN or BN for the evaporation of Al, or made of Al.sub.2 0.sub.3 for the evaporation of Ag, etc. With these crucibles, the evaporand is directly heated by electron bombardment or by eddy currents generated by an induction or work coil. However, even this type of crucible has various disadvantages, chief among them the short life due to the corrosion and erosion caused by high temperature evaporands in direct contact with the ceramic crucible. The same applies to submerged resistor furnaces that are known to be suitable for induction melting of metals, but not for their evaporation which require considerably higher temperatures in the upper part of the furnace.
In addition to their short life, presently used evaporators also suffer from the disadvantage that those parts of the evaporator that do not actively contribute to the evaporation process, i.e. the sides and the bottom of resistance heated boats and also the unwetted inner part and upper rim of e.g. an induction heated crucible, are sources of unwanted radiated heat. This heat is particularly intensive on account of the high emissivity of many of the materials the boats or crucibles are made of. In order to protect heat sensitive targets such as plastics, lacquered paper, etc. from harmful effects of the radiated heat, the evaporator-to-target distance usually equals 100 mm or more. Yet such a considerable distance causes the efficiency, i.e. the percentage of the evaporated metals effectively condensed on the target, to drop to low values, mostly in the order of 30% only, the rest of the evaporated metals or semiconductors being lost on recipient walls and requiring frequent and expensive cleaning.
On the other hand, levitation by electromagnetic forces, coupled with high frequency heating to evaporation temperature, offers the double advantage of exceptional evaporator life and having heat radiated by the levitating evaporand only. According to this method, described by O. Muck in his German Pat. No. 422 004, an electrical conductor is levitated in molten or solid state by the action of electromagnetic forces and can be simultaneously brought to evaporation. Yet a considerable part of the vapor given off by the evaporand condenses on the induction or work coil that induces levitation, so that the latter requires cleaning at frequent intervals. Thus, J. van Audenhove reports in his article: "Vacuum Evaporation of Metals by High Frequency Levitation Heating", J.Sci.Instr., Vol. 36, Mar. 1965, 383-385, that approximately 50% of the evaporated metals condensed on a water cooled work coil and had to be stripped off after breaking the vacuum. It is evident that such condensation, unless prevented in time, does not permit evaporation over several hours.
The present invention is directed to a new method and device for evaporating an evaporand consisting of conducting materials such as metals and semiconductors. The inventive method and device uses the known technique of levitating an evaporand by means of a work coil which may be provided with a boat or other hollow-shaped body. The method includes heating the evaporand to evaporation temperature by electrical means, and also providing for the added new feature of preventing the lasting condensation of the evaporand vapor on said work coil and other parts it has been equipped with. A further new feature of the invention is that very low vapor pressure heavy additives such as tungsten can be added to volatile and lighter weight evaporand such as aluminum, so as to increase the efficiency of high frequency induction heating.